Hi Allison, wouldn't the 1000R-100uF have a time constant of 100 ms?
That would slow my attack speed to 10 times slower than what I think is
the desired range, unless I've got that all wrong!

Thanks for the reference - there certainly is not a lot about this in
the ARRL Handbook.

Tom

On 26 May 2005 11:02:59 -0700, "Tom"
wrote:

Hi Allison, wouldn't the 1000R-100uF have a time constant of 100 ms?
That would slow my attack speed to 10 times slower than what I think is
the desired range, unless I've got that all wrong!

Thanks for the reference - there certainly is not a lot about this in
the ARRL Handbook.

Tom

The series resistance allows overshoot but, limits it. Also it's
series R-C used in addition the paralle C you have for time constant.
EMDRF is the better source than the handbook.

Allision

From: "Tom" on Wed,May 25 2005 10:30 am

I agree with you and slowing the attack is the only way that I have
been able to approach stable operation with a fast release. But 20ms or
longer attack runs counter to what I understand to be the objective -
an attack speed of less than 13 ms and ideally about 1 ms. So, unless I
have this wrong, how do other receivers accomplish similar speeds
without self-oscillation?

The way my circuit operates (I think) is as follows (I'd be happy to
send a schematic to anyone who is interested) :
a) assume an impulse of signal of duration very much longer than the
attack time
b) the rectified signal is filtered of RF by a series-parallel R-C
attack network whose adjustable output feeds an emitter follower
c) the emitter follower pumps current as a low resistance source into
the release R-C network so the attack is not greatly slowed - its
output feeds the AGC driver amp
d) at some point, equilibrium should be reached - the current flow
through the release resistor and AGC driver base should equal the flow
though the emitter follower - but maybe the emitter follower pinches
off and that could be a cause of instability?
e) the signal drops, the attack network discharges at attack speed and
shuts off the emitter follower, so the release capacitor discharges
through its parallel R at release speed, the voltage to the AGC driver
falls so the AGC bias rises at roughly release speed to increase RF/IF
gain.

Having written that out, I have an idea or two I will try.

Having encountered a similar problem many years ago, I'll offer
this as a suggestion: Analyze the behavior of the total signal
amplification chain at LOW frequencies, not at the RF or IF
carrier. Know the control characteristics of the AGC voltage
input to the amplifier versus the total amount of gain of the
receiver chain. Approach the whole receiver AGC action as a
low-frequency servo loop (which is what the AGC actually does).
Think servo control systems theory.

Control systems theory is a rather abstract thing and there
probably will be no sudden bright light of understanding
switched on, but here's a bit of that: The AGC loop action
works by BOTH magnitude and phase at low frequencies.
"Nyquist" and "Bode" plots are helpful there, even though
both of those subjects are also rather abstract. In general,
if the AGC control action results in instability or even motor-
boating, the overall receiver gain - related to the control
voltage range - is too high. Adding a voltage divider at the
low-pass R-C filter of the AGC voltage input will demonstrate
that. Also, the low-frequency phase shifting in the AGC
voltage "decoupling" can upset the phase versus magnitude of
the control voltage. Note: Vacuum tube or FET RF/IF
controlled amplifiers probably use such R-C decoupling, working
only on AGC voltage; other amplifier types might have some
other form of R-C filtering at low frequencies. That low-
frequency magnitude AND phase relationship is important for
total loop stability.

What has to be considered in the AGC loop is the response
through all the decoupling newtorks between the ACG control
source and the controlled device(s). For a "non-linear"
loop (separate attack and decay times) that analysis will be
difficult. It is much easier to analyze with a Spice
simulation that has the capability to model a controlled-gain
amplifier. The whole loop at low frequencies can be modelled
that way. In starting that, forget the RF and IF components
and consider only the amplifications at low frequencies; the
source of the AGC control (detector output) may have to be
modelled slightly differently in that the detector is, in
effect, similar to a power supply rectifier. If that model
is tweaked to be stable with sudden transitions on its input,
then it will be stable at RF and IF.

Let me add one more general note about AGC design. The BFO frequency is
very close to the IF, and it typically puts out volts of signal while
the AGC circuit is trying to operate with millivolts. Unless you're very
careful with layout, shielding, and balance, a lot of BFO signal can get
into the AGC circuit and cause disturbances and malfunctions of various
kinds.

The last AGC circuit I did was very conventional, and it's the sweetest
operating one I've ever used. But I went to great pains to keep the BFO
out of it, and feel that was one of the essential ingredients in getting
it to operate so well.

Roy Lewallen, W7EL

On Thu, 26 May 2005 14:39:43 -0700, Roy Lewallen
wrote:

Let me add one more general note about AGC design. The BFO frequency is
very close to the IF, and it typically puts out volts of signal while
the AGC circuit is trying to operate with millivolts. Unless you're very
careful with layout, shielding, and balance, a lot of BFO signal can get
into the AGC circuit and cause disturbances and malfunctions of various
kinds.

The last AGC circuit I did was very conventional, and it's the sweetest
operating one I've ever used. But I went to great pains to keep the BFO
out of it, and feel that was one of the essential ingredients in getting
it to operate so well.

Roy Lewallen, W7EL

Roy, you have that right. Simple AGC works well but it's the details.
A 6m SSB design I did required a BFO and balanced mixer (DBM prod det)
to be in a sealed box with bypassed feed throughs. Once that was done
the AGC behaved far better.


Allison

From: Roy Lewallen on May 26, 5:39 pm

Let me add one more general note about AGC design. The BFO frequency is
very close to the IF, and it typically puts out volts of signal while
the AGC circuit is trying to operate with millivolts. Unless you're very
careful with layout, shielding, and balance, a lot of BFO signal can get
into the AGC circuit and cause disturbances and malfunctions of various
kinds.

I agree on the need for isolation of various circuits but fail
to see the relevance. A "BFO" is on for OOK CW reception and
normally a manual RF/IF amplification control is used to set a
comfortable listening level. Yes, AGC could be used on OOK CW
but it would be a mistake to derive the AGC control from an AM
detector getting "BFO" input...that would be the same as
introducing a DC bias into the AGC control loop...which would
change the AGC servo-action control...perhaps severely so.

Note: A "BFO" source is steady-state. The detector mixes the
incoming signal (usually at the IF) with the "BFO" to derive
the audio. If the AGC control line is picked off this same
detector, the DC component is akin to having a nearly fixed
DC bias inserted. To use AGC on an OOK CW signal, the audio
tone would have to be used...and that necessaitates a different
sort of AGC control source derivation. A peak-riding, perhaps
selective audio circuit could do that, but the complexity of
that part of the receiving chain is growing. It might be
easier all-around to just pick off the IF to a separate AM
detector as the AGC control line source. The "original"
detector could remain as the OOK CW output with isolated BFO
feeding it.

For SSB voice reception, a "BFO" is still present but a single
diode detector all-purpose sort of detector is far from
optimum as a combined audio source and AGC control line source.
It WILL work, but it is non-linear for both audio and AGC
purposes and that alone could be the source of AGC instability.
It depends on the IF signal level at the detector diode (or
"product detector" which is really a mixer stage).

A single diode with large time-constant on its voltage output
is a peak-riding source for the AGC control line. Whether or
not it follows fast "attack" conditions depends on the source
impedance capabilities of the final IF stage. If that is too
high then the "attack" time is slowed from the necessity to
build up a charge on the diode's load capacitance; that can
be seen on examining an ordinary AC rectifier circuit in
response to a step transient of AC input through various values
of AC source resistors. The peak-riding capability is usually
distorted on the leading edge...which then reflects on the AGC
control characteristics (when loop is closed) in trying to hold
the received signal constant at the detector.

Thought of as a servo-control loop, the AGC subsystem can get
rather involved and complex, affected by a number of different
factors, ALL of which are important insofar as AGC instability
is concerned. "BFO" level is just one item and I will disagree
that it is a very important. It is no more important than
anything else in that loop in my experience.

As a suggestion to anyone else, I would recommend first either
measuring or calculating the AGC control line versus both the
antenna input level and the IF level at the AGC detector input.
That yields a DC baseline datum on the controllable level of
the receiving chain. From that, one can "back-track" calculate
how well the closed-loop AGC action behaves; i.e., the antenna
input RF level versus the peak audio output with AGC on. If
that is using old-style "variable-mu" pentode tubes, then the
control characteristics will show whatever non-linearity it
has steady-state. That can be used as a special controlled-
gain model baseline for a Spice analysis of the AGC loop.
Differing time-constants IN the AGC control feedback can be
set to observe closed-loop response with transient signal
input to the antenna.

The last AGC circuit I did was very conventional, and it's the sweetest
operating one I've ever used. But I went to great pains to keep the BFO
out of it, and feel that was one of the essential ingredients in getting
it to operate so well.

Having had a National NC-57 receiver since 1948, I decided to
"play" with it in 1959 and "improve" its performance, such as
increasing IF gain. The first IF stage as well as the RF stage
were AGC-controlled. Not knowing enough about Control Theory
then, nor considering the low-frequency characteristics of the
AGC control voltage line R-C decoupling, that modification
became a disaster for anything but manual RF gain control. The
motorboating (very low-frequency oscillation) extended to having
the VR-150 screen supply regulator (gaseous shunt regulator to
those of solid-state era times) going on and off. It was
restored to its original components and not played with for over
a decade. Much later, on having had to get into Control Theory
and servo control loops, I could analyze how bad it was and see
what I SHOULD have done. The control was too "tight" in trying
to hold the audio output too constant over a wide signal input
range. There was low-frequency phase shift in the AGC voltage
control decoupling that was responsible for most of the motor-
boating; the VR-150 shunt regulator control range was a bit
too narrow so naturally it had dropped out of regulation and
added the final insult to the original "mod." [forty somethings
and younger may not be familiar with such relaxation oscillator
circuits :-) ]

National Radio Company had made an acceptible product in the
NC-57 but it was a low-end item in their product line. It
worked well enough as a single-conversion HF receiver but it
wasn't optimum in design and no doubt stock logistics at the
factory probably accounted for some of the parts values.
Several passive components seemed to be rather arbitrary in
value choices. I had learned (or should say re-learned) that
NO product is an example is "what something should be" as a
design example.

There just aren't any "easy" answers for some things in
electronics. But, they can be WONDERFUL, challenging
"cross-word puzzle" kinds of thing to solve! :-)

wrote:
From: Roy Lewallen on May 26, 5:39 pm


Let me add one more general note about AGC design. The BFO frequency is
very close to the IF, and it typically puts out volts of signal while
the AGC circuit is trying to operate with millivolts. Unless you're very
careful with layout, shielding, and balance, a lot of BFO signal can get
into the AGC circuit and cause disturbances and malfunctions of various
kinds.


I agree on the need for isolation of various circuits but fail
to see the relevance. A "BFO" is on for OOK CW reception and
normally a manual RF/IF amplification control is used to set a
comfortable listening level. Yes, AGC could be used on OOK CW
but it would be a mistake to derive the AGC control from an AM
detector getting "BFO" input...that would be the same as
introducing a DC bias into the AGC control loop...which would
change the AGC servo-action control...perhaps severely so.
. . .

I apologize for not being more precise in my nomenclature.

By "BFO" I mean the oscillator used for product detection when receiving
SSB and CW signals. No AM detector is involved. The AGC pickoff is of
course done from the IF preceding the product detector, and doesn't
intentionally use the BFO or product detector in any way. The problem I
was alluding to is that the BFO produces a large signal which is very
near the IF, and therefore can get into the AGC circuitry unless some
care is taken to prevent it. This produces a DC bias among other
problems, which can interfere with AGC circuit operation. I found it
necessary to completely shield the BFO, use a good doubly balanced
detector, and use differential amplifiers in the AGC chain in order to
reduce the BFO crosstalk to a tolerable level.

I strongly suspect that a number of the complicated AGC circuits evolved
because a simpler AGC circuit was poorly designed and/or subject to
problems like crosstalk from the BFO. Instead of solving the fundamental
problems, increasingly complex circuits are developed until one
accidentally works correctly, then the improvement is credited to the
complex circuit rather than its accidental relative immunity to the
results of poor fundamental design. This isn't of course universally
true, but it happens pretty often.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Instead of solving the fundamental
problems, increasingly complex circuits are developed until one
accidentally works correctly, then the improvement is credited to the
complex circuit rather than its accidental relative immunity to the
results of poor fundamental design.

I prefer the solution of, "Hmm... there's already a CPU in this radio
anyway... and we've got an ADC around... hey, let's make it the software guy's
problem!"

:-) :-)

From: Roy Lewallen on May 27, 6:49 pm

wrote:

I apologize for not being more precise in my nomenclature.

No problem to me...I fear I got off on a "lecture mode" again,
but was speaking in generalities to other readers about
receiver back-ends.

By "BFO" I mean the oscillator used for product detection when receiving
SSB and CW signals. No AM detector is involved. The AGC pickoff is of
course done from the IF preceding the product detector, and doesn't
intentionally use the BFO or product detector in any way. The problem I
was alluding to is that the BFO produces a large signal which is very
near the IF, and therefore can get into the AGC circuitry unless some
care is taken to prevent it. This produces a DC bias among other
problems, which can interfere with AGC circuit operation. I found it
necessary to completely shield the BFO, use a good doubly balanced
detector, and use differential amplifiers in the AGC chain in order to
reduce the BFO crosstalk to a tolerable level.

Sounds good to me. Separated, isolated detectors allow one to
concentrate on the particulars of each, makes it a lot easier to
work with.

For what it's worth on the audio-output part, I'm more fond of
rather high levels of IF into the detector to get around the
"square-law" response...looking for a better AM envelope
reproduction. While that results in better audio, it also makes
decoupling more difficult to avoid feeding the strong IF back
to the input. Different problem, same cuss-words on the bench,
though. :-)

I strongly suspect that a number of the complicated AGC circuits evolved
because a simpler AGC circuit was poorly designed and/or subject to
problems like crosstalk from the BFO. Instead of solving the fundamental
problems, increasingly complex circuits are developed until one
accidentally works correctly, then the improvement is credited to the
complex circuit rather than its accidental relative immunity to the
results of poor fundamental design. This isn't of course universally
true, but it happens pretty often.

I agree with you there. At least for voice-band detection
receivers (of which I've only built two in a half century from
my own design). Discounting copies of "All-American Five"
table-model cheapies using a single diode for both audio
rectification and (low-pass filtered) for AGC voltage to a
single controlled variable-mu amplifier. Ultimate simplicity
for reasons of price over the counter. One CAN put a BFO on
those (Hallicrafters did back in the late 40s) but the
performance is not the best.

Separating the "detectors" by function is best. The audio
"detector" (I still think of them as 'rectifiers') can be
optimized for best sound. The AGC detector can be optimized
for its action separately...and its response versus IF input
and overall receive chain amplification tailored for the
AGC control-loop "gain." Filtering-decoupling that follows
can be figured out to keep the low-frequency phase response
from upsetting the closed-loop AGC control.

Separate AGC and voice detectors lets one play around with
"attack" and "decay" time-constants with no more than a
single dual op-amp shaping circuit...multiple time-constants
under manual control if desired, that won't interfere with
the audio detection part. AGC detector input would have to
be the fastest-responding (to desired time-constant) with
a relatively simple op-amp doing the time-stretching.

Some folks might consider that op-amp addition "complicated."
Won't blame them if they do. From my experience, a
"complicated" AGC subsystem is having to AGC on a
1 uSec pulse with a time gate in the presence of other
assynchronous 1 uSec pulse sidebands located on 1 MHz
intervals (up to 3) on either side...with a decay to attack
time ratio of about 1000:1. :-) Did that for an R&D
airborne system at RCA...was somewhat too much but that
allowed a greater simplification for a following generation
of airborne equipment. A lesson there can be to "cover all
bases possible" the first time around, then investigate to
see what can be simplified for something less complicated.

AGC, in the basic consideration, should begin as a control
loop. From there on its a matter of choice of circuits.

Roy Lewallen wrote:
I strongly suspect that a number of the complicated AGC circuits evolved
because a simpler AGC circuit was poorly designed and/or subject to
problems like crosstalk from the BFO. Instead of solving the fundamental
problems, increasingly complex circuits are developed until one
accidentally works correctly, then the improvement is credited to the
complex circuit rather than its accidental relative immunity to the
results of poor fundamental design. This isn't of course universally
true, but it happens pretty often.

All too many software wannabees work this way too.

--
I miss my .signature.